1. Field of the Invention
The present invention relates to a fundus examination apparatus for examining blood vessels on patient""s fundus in ophthalmic hospitals or the like.
2. Related Background Art
Conventionally, a fundus hemodromometer or blood flow meter irradiates a blood vessel on the fundus of an eye to be examined with a laser beam having a wavelength xcex, receives scattered reflected light by a photodetector, detects an interference signal between a Doppler shift component as the scattered reflected light from the bloodstream in the blood vessel and scattered reflected light from the blood vessel and analyzes the frequency of the signal to obtain the blood flow velocity.
The blood flow velocity can be obtained from a maximum velocity Vmax given by:
Vmax={xcex/(nxc2x7xcex1)}xc2x7||xcex94fmax1|xe2x88x92|xcex94fmax2||/cosxcex2xe2x80x83xe2x80x83(1)
where xcex94fmax1 and xcex94fmax2 are the maximum shifts of frequencies calculated from light-receiving signals obtained by two light-receiving devices, n is the refractive index at the measurement portion, xcex1 is the angle between the two light-receiving axes in an eye, and xcex2 is the angle between the plane formed by the two light-receiving axes in the eye and the velocity vector of blood flow.
When measurement is done from two directions, contributions of measurement light in the incidence directions are canceled out, and the blood flow velocity at an arbitrary portion on the fundus can be measured. In addition, when the line of intersection between the plane formed by two light-receiving axes and the fundus is made to coincide with the angle xcex2 the plane makes with the velocity vector of the bloodstream, the actual maximum blood flow velocity can be measured because xcex2=0xc2x0.
In such a fundus hemodromometer for measuring the blood vessel shape or blood flow velocity at a specific portion of a fundus blood vessel using a laser beam, the measurement portion must be accurately irradiated with the measurement light beam during the measurement processing time. However, actually, it is difficult to continuously accurately irradiate the measurement portion with the measurement light beam because of involuntary eye movement of the eye to be examined. Apparatuses having a tracking means for detecting a blood vessel position and moving the measurement light beam irradiation point on the measurement portion in real time in correspondence with involuntary eye movement are disclosed in, e.g., Japanese Laid-Open Patent Application No. 63-288133 and PCT Laid-Open No. 6-503733.
In these ophthalmic apparatuses, a linear CCD is used as a light-receiving means for receiving a tracking light beam reflected by the fundus to process the waveform of a blood vessel image signal. Tracking is performed by calculating the displacement amount between the tracking reference position and blood vessel image position signal. The fundus is irradiated with tracking light and measurement light through mirrors set at the pupil conjugate positions. When a plurality of blood vessel image position signals are present, tracking may be controlled on the basis of a blood vessel position signal closest to the tracking reference position.
In the above prior arts, however, when a thick blood vessel with high contrast is selected as an object to be measured, regular reflected light from the blood vessel wall is observed at the center of the blood vessel, and an image signal is observed as if there were two blood vessels. In this case, two or more blood vessel position signals are present. For this reason, when tracking is controlled on the basis of the blood vessel position signal closest to the tracking reference position, the blood vessel position closest to the tracking reference position changes due to involuntary eye movement of the eye to be examined, and tracking becomes unstable.
It is an object of the present invention to solve the above problem and provide an apparatus capable of accurately detecting a blood vessel.
It is another object of the present invention to provide a fundus examination apparatus for easily and accurately measuring fundus blood vessel information by accurate tracking.
A blood vessel detecting apparatus of the present invention is characterized by comprising:
image pickup means for picking up an image of a blood vessel;
detection means for detecting a portion estimated as a blood vessel on the basis of an output from said image pickup means; and
determination means for, when said detection means detects a plurality of portions that are likely to be blood vessels, determining the correct number of blood vessels on the basis of the output.
Especially, when the detection means detects a plurality of portions that are likely to be blood vessels, and the portions that are likely to be blood vessels are present within a predetermined width, the determination means determines that the number of blood vessels is one, and when the portions that are likely to be blood vessels are separated by a distance greater than the predetermined width, the determination means determines that the number of blood vessels is two.
There is also provided a blood vessel detecting apparatus comprising:
an area sensor for picking up an image of a blood vessel; and
a processing circuit for executing a signal processing operation to extract a feature point of the blood vessel on the basis of an image output from the area sensor concerning a direction perpendicular to a flowing direction of blood in the blood vessel, formed on the area sensor.
Other objects, features, and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.